Adsorption of phenol using adsorbent derived from Saccharum officinarum biomass: optimization, isotherms, kinetics, and thermodynamic study

The present research shows the application of Taguchi's design of experiment approach to optimize the process parameters for the removal of phenol onto surface of Saccharum officinarum biomass activated carbon (SBAC) from an aqueous solution to maximize adsorption capacity of SBAC. The effect of adsorption parameters viz. adsorbent dose (m), temperature (T), initial concentration (C0) and mixing time (t) on response characteristics i.e., adsorption capacity (qt) has been studied at three levels by using L9 orthogonal array (OA) which further analyzed by variance analysis (ANOVA) for adsorption data and signal/noise (S/N) ratio data by using ‘larger the better’ characteristics. Using ANOVA, the optimum parameters are found to be m = 2 g/L, C0 = 150 mg/L, T = 313 K and t = 90 min, resulting in a maximum adsorption capacity of 64.59 mg/g. Adopting ANOVA, the percentage contribution of each process parameter in descending order of sequence is adsorbent dose 59.97% > initial phenol concentration 31.70% > contact time 4.28% > temperature 4.04%. The phenol adsorption onto SBAC was best fitted with the pseudo-second-order kinetic model and follows the Radke-Prausnitz isotherm model. Thermodynamic parameters suggested a spontaneous, exothermic nature and the adsorption process approaches physisorption followed by chemisorption. Hence the application of Taguchi orthogonal array design is a cost-effective and time-efficient approach for carrying out experiments and optimizing procedures for adsorption of phenol and improve the adsorption capacity of SBAC.


Preparation and characterization of SBAC
Saccharum officinarum (sugarcane) is one of the major commercial crops in India and it is planted thrice a year in October, March and July depending on the part of the country without the addition of fertilizers and under a controlled watering regime 31 .Sugar cane bagasse is a lignocellulosic fibre residue obtained from sugar cane culm after the culm is milled and the juice is extracted.The average composition of sugar cane is 65-75% water, 11-18% sugars, 8-14% fibres and 12-23% soluble solids.The cane basically consists of juice and fiber 32 .The sugar cane bagasse has the following composition (by weight): cellulose, 41.8%; hemicellulose (as pentosane), 28.0%; lignin, 21.8% 33 .In the present study, agricultural and agro-industrial waste materials like sugarcane bagasse were obtained from Mahatma Phule local market of Nagpur district, Maharashtra, India.The material was undergone thorough cleaning with tap water to eliminate the soil/residue, and afterwards it was subjected to sun drying for about 48 h.The dried sugarcane bagasse was grounded into fine particles and sieved for acquiring particles in the range of 850-300 µm.Sieved material was blended with 0.1 M zinc chloride solution (ZnCl 2 ) in 1:0.25 (g:mL) proportion.This mixture was placed in muffle furnace for carbonization at a temperature of 673 K for 60 min.The obtained charred sample was washed many times by doubled distilled water (DDW) to make it free from the acid and dried at 378 K for about 3 h.The dried carbon passing through 600 µm sieve and retaining on 150 µm sieve (ASTM 11-70) was utilized for adsorption study" 26 .The proximate analysis of adsorbent was performed according to the BIS 1350-1 (1984) standard."Scanning electron microscopy (SEM) was carried out using a JSM-7610F instrument from Japan to examine the surface morphologies of SBAC.Fourier transform infrared spectrometry (FTIR) was employed using an IR Affinity-1 instrument (Miracle 10, Shimadzu, Japan).The FTIR analysis covered the spectral range of 400-4000 cm −1 to study the surface functional groups present on SBAC.The Brunauer-Emmet-Teller (BET) method is used to measure the surface area and pore characteristics of SBAC.Nitrogen adsorption at 77 K was performed using a Quantachrome Nova touch 1.1analyzer." Approximately 0.35 g of dry powder sample was degassed for about 2 h at 200 °C before conducting the BET analysis.Origin Pro (2022b) software was used for the processing of the raw data obtained from the characterization.

Taguchi's method
The Taguchi's L 9, orthogonal array (OA) is used to optimize the adsorption parameters affecting the adsorption capacity (q t ) of SBAC.The controllable process parameters considered are adsorbent dose, initial phenol concentration, temperature, and mixing time, denoted as factors A, B, C, and D, respectively.Each of these parameters is tested at three different levels, labeled as L1, L2, and L3 shown in Table S2.The study did not consider two other parameters, initial pH and agitating speed, as they were previously found to have minimal influence on the phenol adsorption onto SBAC 26 .To determine the number of experiments needed for optimization, the authors applied the partial factorial method.According to this method, the number of experiments, denoted as E, can

Experimental data analysis
ANOVA (Analysis of Variance) and S/N (Signal-to-Noise) ratio are used to analyse the data collected from the experimental study.The S/N (signal-to-noise) ratio is used to assess the impact of uncontrollable factors, also known as noise factors 34 .states that the S/N ratio helps reduce the variability caused by these noise factors.The S/N ratio is a measure of the quality of a function or process.The larger the ratio, the better the function performs relative to the noise or errors present.Three types of S/N ratio which are 34 : Smaller the better

Larger the better
Nominal the better Equation ( 3) is used to find the larger the better S/N ratio in experimental design and analysis.It quantifies the relationship between the mean (μ) and variance (σ 2 ) of the observed response variables (y i ) for a given number of trials (n).After calculating the S/N ratios, the data are often further analyzed using analysis of variance (ANOVA) techniques.When applying ANOVA to experimental data, an orthogonal array (OA) can be used to allocate factors to experimental trials efficiently.Each column of the OA represents a specific factor and its associated degrees of freedom (DOF), which is equal to the number of levels minus one.The array must meet the criterion that the total DOF of OA is more than or equal to the total DOF of the experiment.The methodology for analyzing experimental data is described in detail 35,36 .ANOVAs analysis conducted on raw data to find out the optimum conditions and the mean of response (μ), for example A, B, C, and D are the parameters optimized at one, three, two and one level respectively then µ calculates using the formula: whereas T is the grand total of all results, N is the total number of results; A 1avg , B 3avg , C 2avg and D 1avg are the average response values.
Based on the average outcomes of the studies, µ is calculated.The optimized result was examined using a confidence interval (CI), which shows where a statistical parameter's value falls in relation to where it should fall given a certain degree of confidence.The CI is classified into two categories: CI CE , which is only applicable to a sample group or set of experiments under certain circumstances, and CI POP , which is the confidence interval for the whole population 30 .
(1) where R is the sample size for confirmation experiment, as R approaches infinity, meaning the sample size approaches the entire population (N), the term 1/R approaches zero.This implies that the influence of the sample size on the calculation of the confidence interval diminishes, and the CI CE converges to the Confidence Interval for the Population (CI POP ).On the other hand, as R approaches 1, indicating a very small sample size relative to the population, the term 1/R becomes large.Consequently, the CICE becomes wider, indicating a larger margin of error and greater uncertainty in the estimate of the population proportion 37,38 .The confirmation test will be performed at optimal conditions for parameters, with the average of the results confirmed at 95% confidence intervals.If Taguchi's model value and confirmation experiment value are within the 95% confidence interval, this confirms the validity of the optimum parametric values obtained from Taguchi's design.

Adsorbent characterization
The N 2 adsorption-desorption tests using the BET-BJH method indicate a type-II isotherm, which suggests a multilayer adsorption phenomenon.This information is derived from Fig. 1.Before adsorption, the surface area of the SBAC is measured to be 415.96m 2 /g.However, after adsorption, the surface area decreases significantly to 78.33 m 2 /g.The reduction in surface area is approximately 337.63 m 2 /g or 81.16% of the initial value.The efficient adsorption of phenol is attributed to the observed surface area and microporous structure of the SBAC.This indicates that the SBAC material is capable of effectively capturing and removing phenol through adsorption process.
The proximate analysis of adsorbent material was conducted according to the BIS 1350-I (1984) standard.The results of this analysis are shown in a Table S4.The analysis provides important information about the composition of the adsorbent, which can be useful in determining how to manage the material after it has been used for the adsorption process.Lower moisture content and volatile substance are desirable because it can increase the calorific value of the material.The amount of ash content can provide insights into the management of the ash after the adsorbent has been used.The high content of fixed carbon in the adsorbent indicates a better calorific value, making it suitable for incineration or disposal methods that involve burning.However, it's important to consider the emissions of potentially toxic substances during incineration to ensure proper environmental management 4 .
The results of the scanning electron microscopy (SEM) study showed that the surface of the material under investigation exhibited a heterogeneous morphology, characterized by the presence of numerous pores.These pores tended to enhance the adsorption process, suggesting that they played a significant role in the material's ( 7)  www.nature.com/scientificreports/ability to adsorb substances.This is supported by the observations made in Fig. 2A,B.Interestingly, after the adsorption of phenol, the pore structure associated with the high surface area appeared to disappear, as shown in Fig. 2B.Instead, a smoother and more even surface was observed.This suggests that the adsorption of phenol led to changes in the surface morphology of the material, potentially due to the filling or blocking of the pores by the adsorbed phenol molecules 39 .The introduction of ZnCl 2 chemical activation caused the formation of new micropores and the expansion of existing micropores.This resulted in an increase in the volume and surface area of the micropores.During the carbonization process, the pores were generated as a result of the evaporation of ZnCl 2 .The activation with ZnCl 2 led to the development of a significant number of micropores, which played a crucial role in the removal of pollutants from the solution 40 .
The FTIR spectrum before and after phenol adsorption on the surface of SBAC revealed certain changes in the spectral bands as shown in Fig. 3.Here is a breakdown of the observed changes and their corresponding functional groups: these dips in the spectrum at 3856 and 3714 cm −1 indicate the presence of -OH functional groups, specifically phenol, alcohol, and carboxylic acid groups 41 , dip at 2356 cm −1 corresponds to the O-C=O functional group 42 , dip at 1681 cm −1 attributed to the vibrations of C=O functional groups [43][44][45] , dip at 1542 cm −1 is associated with aromatic C=C bonds 46 , dip at 1175 cm −1 represents the ν C-O vibrations of phenols and ethers 47 , dips at 874 cm −1 , 808 cm −1 , 746 cm −1 , 679 cm −1 are signifies the presence of isolated hydrogen, two adjacent hydrogens, four adjacent hydrogens, and five adjacent hydrogens respectively 48,49 .These observations in the FTIR spectrum before and after adsorption provide valuable information about the functional groups present in SBAC and the changes that occur as a result of the adsorption process.
The pH at which adsorbent surface posses zero charge know as point of zero charge (PZC).PZC of SBAC takes place at pH 6 as shown in Fig. 4A.Decreasing the pH < 6 causes positive charge and increasing the pH > 6 causes adsorbent surface negative on the adsorbent surface.Phenol was a very weak acid with the pK a value of 9.89, which was dissociated if the pH value exceeds pK a and at low pH it was mainly in molecular state as shown in log C-pH graph Fig. 4B.With higher pH values, the removal of phenol decreases because of phenol ionization and repugnance between phenolate anions and negative SBAC sites 50 .At lesser pH (< PZC), phenol molecules get easily attached onto the negative SBAC surface and moreover the pH of the phenol less than the PZC i.e., 5.5 < 6.As a result, at pH 5.5 the maximum removal of phenol takes place 4 .

L 9 Orthogonal array experimental results
Batch study is carried out for the removal of phenol onto SBAC at particular conditions which are listed in the Table S3.Three times each experiment is conducted and average q t value is employed for the Taguchi's analysis.In Table S5, the results of experimental runs, including S/N ratio values determined under larger the better conditions, are reported.

Effect of adsorption parameters
Adsorption capacity is the primary response characteristic for optimization, and these values are controlled by process factors like m, C 0 , T, and t given in Table S5 at various levels.The fact that factor A had the greatest effect at level 1 means that altering the value of parameter A at level 1 had a considerable impact on the q t values, as shown by Fig. 5 and Table S6.However, for the adsorbent SBAC, variables B, C, and D had the greatest influence at level 3.The influence of each level relative to the others was determined by comparing levels 1 and 2 (L 2 -L 1 )  www.nature.com/scientificreports/and levels 2 and 3 (L 3 -L 2 ).A larger disparity between two levels signifies a higher influence of that level.In this case, Table S6 suggests that parameter B (C 0 ) had the greatest influence on the q t values.The q t values increased as C 0 increased, which can be attributed to a higher mass driving force and a lesser barrier to the absorption of phenol onto SBAC 37 ). Figure 5 shows that as the levels of parameter A (m) increase from level 1 to 3, denoted as m, the sorption capacity decreases from 44.04 to 12.21 mg/g.This decrease is attributed to a decrease in the phenol to SBAC ratio.In other words, as the level of parameter A increases, there is a larger amount of adsorbent available compared to the amount of phenol, resulting in a lower sorption capacity.As the levels of parameter B (C 0 ) increase from level 1 to 3, the sorption capacity increases from 12.97 to 37.27 mg/g.This increase is attributed to an increase in the mass transfer driving force and a lesser barrier to the absorption of phenol onto SBAC.In simpler terms, higher levels of parameter B create conditions that facilitate the sorption process, such as enhancing the mass transfer of phenol molecules onto SBAC and reducing barriers to the adsorption process.As the levels of parameter C (T) increase from level 1 to 3, the sorption capacity increases from 21.47 to 29.95 mg/g.Higher temperatures result in lower phenol viscosity, increased molecule mobility, and higher kinetic energy.These conditions enhance the chances of phenol molecules adsorbing onto SBAC and increase the diffusion rate of phenol, leading to an increased sorption capacity.The sorption capacity of phenol onto SBAC changes as the parameter D (t) increases from level 1 to 3. The sorption capacity of SBAC first increases quickly from level 1 to 2, (20.19 to 26.49 mg/g).This increase is attributed to the abundance of large unoccupied sites on SBAC for the sorption of phenol.i.e., when the contact time is increased from level 1 to 2, more vacant sites become accessible for phenol molecules to adsorb onto SBAC, resulting in a higher sorption capacity.However, in the later phase, as the contact time increases further from level 2 to 3 (26.49to 28.84 mg/g), the availability of vacant sites on SBAC decreases.This reduction in available vacant sites hampers the adsorption capacity of phenol, causing a retardation in the sorption process.Consequently, the increase in adsorption capacity from level 2 to 3 for SBAC is only 2.35 mg/g, which is relatively low compared to the previous increase observed.
The percentage contributions of a different parameters to the overall sorption of phenol over SBAC are shown in Fig. 6 and Table S7.The parameter A (m) has the highest contribution of 59.97% to the sorption process.It indicates that the amount or dosage of the adsorbent plays a significant role in the sorption of phenol as increasing the adsorbent dose can enhance the sorption efficiency.The second most influential parameter is B (C 0 ) with a contribution of 31.70%.This suggests that the concentration of phenol in the initial solution affects its sorption onto SBAC.Higher initial concentrations may result in reduced sorption efficiency.The parameter D (t) has a relatively lower contribution of 4.28%.This indicates that the contact time between the phenol solution and SBAC influences the sorption process to a lesser extent compared to factors A and B as longer contact times may lead to increased sorption.The parameter C (T) with the least effect on the sorption process with a contribution of only 4.04%.This suggests that temperature plays a minor role in the sorption of phenol onto SBAC.However, it's important to note that even though its contribution is low, temperature can still impact the kinetics and thermodynamics of the sorption process.

Selection of optimized level and estimation of optimized response characteristics
Optimization study was conducted to determine the optimal levels of parameters A, B, C, and D for maximizing the response value (q t ) in a process.The study involved analyzing a response curve (Fig. 5) and Table S6 to identify the factor levels that produced the highest q t values for SBAC.The results indicate that the first level of Parameter A (m) and the third level of parameters B (C 0 ), C (T), and D (t) yielded maximum values of q t .The average values of q t (mg/g) for these optimal levels are as A 1 = 44.04,B 3 = 37.27, C 3 = 29.95 and D 3 = 28.84.The grand total of all the qt results (T) is reported as 679.66 mg/g, and the total number of results (N) is given as 27.These values provide an overview of the performance of the optimization study and the distribution of q t values obtained.Put the all these values in Eq. ( 6) becomes µ SBAC is 64.59 mg/g.Keep values N is 27 and total DOF associated in the estimate of mean is 2 in Eq. ( 9), becomes n eff is 3. To calculate the confidence interval of 95% for the population mean and the confirmation experiments, certain values are required such as N is 27, the Figure 6.The percentage of each parameter's contribution to the adsorption capacity of SBAC for phenol.
of the earlier studies reports that optimum time is 30 min for ZnO Nano catalyst 10 , 60 min for banana peels 4 , 120 min for guava tree bark 20 , and 120 min wood charcoal 61 .
In the present study, pseudo-first-order and pseudo-second-order kinetic models 62 are used to analyze the kinetic data and presented in Table S10.When these models are compared, the regression value obtained for the pseudo-second-order model (R 2 = 0.9999) is comparatively higher than that first order which suggests that pseudo-second-order kinetic model suitable for the adsorption phenol onto SBAC (Fig. 8B).Some earlier studies also cited the pseudo-second-order model as the best fit model for the phenol removal on chitin 63 , babul saw dust 23 , and scoria stone 16 .

Adsorption thermodynamics
Thermodynamic analysis is carried out for the phenol adsorption by increasing the temperature from 283 to 323 K while maintaining the same contact time of 90 min, adsorbent dosage of 2 g/L, and initial phenol concentration of 150 mg/L, phenol pH of 5.5 and agitation speed of 150 rpm.Thermodynamics parameters can be calculated out by using distribution coefficient, K ads which is dependent on temperature.In our study, the Radke-Prausnitz is found to be the best fit isotherm.Hence, the values of constant K RP viz.0.095, 0.258, 0.182, 0.148 and 0.174 L/g at respective temperatures of 283, 293, 303, 313 and 323 K are calculated from the Radke-Prausnitz plot as shown in the Table S11.
The Gibbs free energy change equation 64 Equating the above two equations, we get where, ΔG 0 ads Gibbs free energy change (kJ/mol), T Temperature (K), ΔS 0 change of entropy (J/mol K), R Universal gas constant, ΔH 0 change of enthalpy (kJ/mol), and K ads distribution coefficient (L/mol).A plot of ln K ads against 1/T, as illustrated in Fig. 9, gave a linear relationship with ΔH 0 and ΔS 0 evaluated from the slope and intercept, respectively, of the Van't Hoff plot and ∆G 0 ads was calculated using Eq. ( 12).The thermodynamic parameters for the adsorption of phenol onto SBAC are shown in Table S11.
The values of ΔG 0 ads fall within the range of − 19.599 to − 23.222 kJ/mol.These values suggest that the sorption process approaches physisorption (physical adsorption) followed by chemisorption (chemical adsorption).Physisorption is indicated by ΔG 0 ads values in the range of 0 to − 20 kJ/mol, while chemisorption is indicated by values in the range of − 80 to − 400 kJ/mol.It is observed that the values of ΔG 0 ads become more negative (higher in magnitude) with an increase in temperature.This indicates that the driving force for the sorption process increases at higher temperatures.The negative values of ΔG 0 ads obtained in the analysis indicate that the sorption process is thermodynamically favorable for the adsorption of phenol onto SBAC 65 .The values of ΔH 0 , the standard enthalpy change, were determined to be negative (− 6.035 kJ/mol).This suggests that the sorption process is exothermic.The values of ΔS 0 , the standard entropy change, were found to be positive (0.09 kJ/mol K).This indicates that the adsorption process leads to an increase in the stability of the adsorption phase.Positive ΔS 0 values imply greater randomness or disorder at the molecular level, contributing to the overall stability of the adsorbed species.

Adsorption mechanism
The feasible mechanism is based on a thorough understanding of all conceivable physical and chemical interactions between the substrate (phenol) and adsorbent (SBAC), as evidenced by characterization data and experimental results.Due to van der Waals forces, chemical affinity, electrostatic attraction and a number of different functional groups, such as hydroxyl, carbonyl, lactone, ketone, aldehyde, etc., are present on the surface of activated carbon and contribute to the formation of physical and chemical bonds with phenol during the adsorption process 66 .The emulous behavior of ZnCl 2 -treated activated carbon could be attributed to additional chemical interactions via Lewis acid catalyzing reactions (Fig. 10) between nucleophilic groups and aromatic hydrocarbon, ketone or aldehyde groups 67 .

Conclusions
The present study focuses on using Taguchi's design of experiment approach to optimize the processes parameters for the removal of phenol onto SBAC from an aqueous solution.Batch experimental results shows that the highest adsorption capacity of SBAC achieved around 64.59 mg/g under the optimum conditions is found (m = 2 g/L, C 0 = 150 mg/L, t = 90 min and T = 313 K).The ANOVA study shows the adsorbent dosage has maximum influence (59.97%) and temperature has minimal influence (4.04%) on adsorption capacity of SBAC for the removal of phenol.For SBAC, the optimized parameters at various levels were determined to be A 1 , B 3 , C 3 , and D 3 and the q t value given by the Taguchi's approach and the experiment is almost same i.e., 64.59 mg/g.Therefore, the confirmation experiment (run number 3) shows that the adsorption capacity of SBAC values within the range  www.nature.com/scientificreports/ of 95% CI CE .An isotherm study shows that Radke-Prausnitz isotherm model is well fitted to the equilibrium data obtained from experimental programme.Kinetic study raveled that pseudo-second-order kinetic model exhibited the best fit to the experimental data.Thermodynamic study (ΔH 0 = − 6.035 kJ/mol and ΔS 0 = 0.090 kJ/ mol K) shows that the nature of adsorption process is random, spontaneous, exothermic and (ΔG 0 ads = − 19.599 to − 23.222 kJ/mol) suggesting that the adsorption process approaches physisorption followed by chemisorption.The BET surface area of SBAC before and after phenol adsorption are 415.96and 78.33 m 2 /g, and observed surface area decreased by 81.17% after adsorption indicating the better sorption capacity SBAC.SEM study showed that the surface of adsorbent exhibited a heterogeneous morphology, characterized by the presence of numerous pores improves the adsorption, and proximate analysis of adsorbent determined consists 65.33% of fixed carbon improves the potential of the adsorbent.FTIR study shows that presence of hydroxyl, carbonyl, lactones, ketone, and aldehyde groups are responsible for phenol removal.Hence the application of Taguchi orthogonal array design is a cost-effective and time-efficient approach for carrying out experiments and optimizing procedures for adsorption of phenol by SBAC.

Figure 1 .
Figure 1.(A) N 2 adsorption-desorption isotherms for before and after adsorption and (B) pore volume versus pore size distribution graph of SBAC before and after adsorption.

Figure 5 .
Figure 5.Effect of adsorption parameters on adsorption capacity of SBAC and S/N ratio for phenol removal onto SBAC.